Why are we pretending every journey in London is a 5 minute hop on, hop off. Many people are forced to make long journeys by bus because they can't afford train fares, (such as those on long routes such as the x26, x68, 53 and others where you have to instead change routes as there isn't a direct long one). I don't think there's a desperate need to install WiFi (I'd prefer TFL to focus on introducing battery technology to improve air quality in London, not just central london too) but I think it's something that does make you sort of go 'oh ok wow' if you're visiting such a large, wealthy capital.
Saying that, if they used the same system as tube station WiFi where you have to sign in with your provider, it wouldn't be very useful to tourists from other countries anyway, the WiFi they've installed on the 710s and 345s is much better.